Seasonal Polar Carbon Dioxide Frost on Mars: Spatiotemporal Quantification of CO2 Utilizing 2001 Mars Odyssey Gamma Ray Spectrometer Data

by Kelly, Eleanor Jane.

The exchange of carbon dioxide between the atmosphere and the polar caps on Mars creates a seasonal cycle of growth and retreat of the polar caps. As the major component of the Martian atmosphere, CO2 condenses in the polar regions of the planet during the winter seasons and precipitates as CO2 frost. It then sublimes during the spring and summer seasons in response to solar radiation. Through natural radioactivity or when exposed to cosmic rays, elements in the Martian near-subsurface (uppermost meter) emit gamma rays with distinct, characteristic energies. The Gamma Ray Spectrometer (GRS) onboard the 2001 Mars Odyssey satellite is used to measure the gamma rays coming from the Martian regolith to calculate elemental distributions, abundances, and temporal variations in the gamma ray flux. Changes in the CO2 frost over time can be quantified by observing attenuation effects of H (2223 keV hydrogen) and 40K (1461 keV potassium) gamma ray signals transmitted through various depths of polar CO2 overburden throughout the Martian seasons. Conclusions are drawn about the spatial extent, column density, and mass of Mars’ seasonal polar caps as a function of time utilizing GRS data. Columnar thickness and mass results are discussed and plotted for latitudes including ±60o and poleward. GRS observations are compared to predictions from the NASA Ames Research Center Mars General Circulation Model (ARC GCM) and to similar experimental results from the Mars Odyssey High Energy Neutron Detector (HEND) and the Neutron Spectrometer (NS). Models for north and south polar atmosphere and regolith distributions are incorporated, and the results indicate that the assumption of a 100% H2O-ice residual cap underlying the seasonal frost in the north is accurate. The GRS CO2 frost observations are in good agreement with the other studies mentioned, in particular for the timing of the beginning of frost deposition to the 13 complete sublimation of surface CO2 back into the atmosphere. The total amount of condensed CO2 mass derived from GRS data is on the order of 6.0 × 1015 kg and verifies previous reports that ?25% of the total Martian exchangeable-CO2 reservoir participates in the ground-atmosphere cycle. 14